The present invention relates to automobile aftermarket circuitry such as car alarms, door lock and starter control circuits.
Consumers frequently seek to upgrade their vehicle by adding various electronic functions not included in the vehicle""s original equipment. Often, consumers desire an aftermarket upgrade enhancing the electronic functionality of the vehicle, for example adding a car alarm, a door lock control system permitting remote control of door locks, and/or a starter control system permitting remote starting of the vehicle""s engine. A large number of aftermarket systems are presently available to add these various functions to vehicles.
Aftermarket electrical systems typically include power and ground connections for connection to the vehicle""s battery, either directly or through the ignition switch. The electrical system derives power from these power and ground connections, and uses this power to drive electrical components either directly or using relays. As one example, an aftermarket remote starter control can include a receiver for receiving radio signals indicating that the vehicle is to be started, and a current output for driving a relay wired in parallel with the vehicle""s ignition switch for delivering current from the battery to the vehicle""s starter.
Aftermarket electrical systems of this kind have proven to be difficult to install. Correct electrical connections must be made to the vehicle""s battery and other electrical systems. Unfortunately, these connections are not always made correctly. In a surprisingly large percentage of cases, which can be as large as 35% of all installations, the connections from the aftermarket electrical system to the vehicle are made incorrectly, causing damage to the aftermarket electrical system. For example, the negative and positive connections from the aftermarket electrical system must be correctly wired to the vehicle""s battery. If the vehicle""s battery is mistakenly connected to an output lead, excessive current will flow through the aftermarket electrical system, destroying the output circuitry.
Accordingly, it is an object of the present invention to improve aftermarket electrical systems by incorporating protective components which will reduce the likelihood of damage when incorrect connections have been made to the electrical system.
In accordance with the present invention, this object is accomplished by incorporating, between output circuitry of an aftermarket electrical system and the output terminal, a current limiting circuit capable of dissipating power greater than about xc2xc Watt. By incorporating such a current limiting circuit, the output circuitry is protected from excessive currents flowing through the output terminal as a consequence of incorrect connections being made to the output terminal.
In the specific embodiment described below, the current limiting circuit is a resistor, and the resistor has a value of approximately forty Ohms, and a power dissipation rating of three Watts. When the output terminal is connected to a typical relay winding and the output circuitry is energized, a resistor of this value and power rating will permit a current flow of approximately ninety milliAmps through the relay winding, which is sufficient to activate the relay. Thus, the resistor does not interfere with normal operation of the aftermarket electrical system when correctly connected. However, if the battery voltage is inadvertently connected directly to the output terminal, a resistor of this value will limit the current flow into the output circuit to no more than about 300 milliAmps, which is substantially less likely to damage the output circuitry than the much greater currents that would be generated in the absence of an output resistance.